1. Field of the Invention
The present invention relates to a solid state radiation detector having a capacitor unit for storing, as latent image charges, charges of an amount in response to a dosage of radiation irradiated thereto or an amount of light generated by excitation by the radiation.
2. Description of the Related Art
Presently, in radiation imaging for the purpose of medical diagnosis and the like, there are proposed various types of radiation image information recording/reading apparatuses using a solid state radiation detector (hereinafter, simply referred to as a detector in some cases). In the solid state radiation detector, charges obtained by detecting radiation are stored as latent image charges in a capacitor unit, the stored latent image charges are converted into electric signals representing radiation image information, and the electric signals are outputted. Various types of the solid state radiation detectors for use in these apparatuses are proposed. Among them, there is one of an optical reading system, in which reading light (reading electromagnetic wave) is irradiated onto a surface of the detector, from which the stored charges are read out to the outside in a charge reading-out process, and the charges are read out.
In Japanese Unexamined Patent Publication Nos. 2000-105297, 2000-284056 and 2000-284057, there are proposed a detector as will be described below as a solid state radiation detector of the optical reading system, which is capable of combining high-speed responsiveness of the reading out with high efficiency in extracting signal charges. Specifically, there is proposed a detector including: a first conductive layer having a transmissivity to light generated by recording radiation or excitation by the radiation (hereinafter, referred to as recording light); a recording photoconductive layer exhibiting a conductivity by receiving the recording light; a charge transport layer operating approximately as an insulator for charges of the same polarity as that of charges charged in the first conductive layer and operating approximately as a conductor for charges of a polarity reverse to that of the charges of the same polarity; a reading photoconductive layer exhibiting a conductivity by receiving irradiation of the reading light; and a second conductive layer having a transmissivity to the reading light, the detector being composed by stacking the above-described constituents in accordance with the order listed above, wherein signal charges (latent image charges) bearing image information are stored in a capacitor unit formed on an interface between the recording photoconductive layer and the charge transport layer.
Moreover, in the above-described Japanese Unexamined Patent Publication Nos. 2000-284056 and 2000-284057, particularly, there is proposed a detector as will be described below. In the detector, stripe electrodes are used for electrodes of the second conductive layer having a transmissivity to the reading light, the stripe electrodes being composed of optical charge pair generating linear electrodes having a transmissivity to a large number of reading light. Moreover, sub-stripe electrodes are provided in the second conductive layer so as to be alternate with the optical charge pair generating linear electrodes (stripe electrodes) and to be parallel thereto. The sub-stripe electrodes are composed of a large number of optical charge pair non-generating linear electrodes for outputting electric signals on a level in response to an amount of the latent image charges stored in the capacitor unit.
As described above, the sub-stripe electrodes composed of the large number of optical charge pair non-generating linear electrodes are provided in the second conductive layer. Thus, a new capacitor is formed between the capacitor unit and the sub-stripe electrodes, and thus charge rearrangement during the reading can charge the sub-stripe electrodes with transported charges of the polarity reverse to that of the latent image charges stored in the capacitor unit by the recording light. Thus, an amount of the transported charges distributed through the reading photoconductive layer to the capacitor formed between the stripe electrodes and the capacitor unit can be reduced to be relatively smaller than the case without providing these sub-stripe electrodes. Consequently, the amount of signal charges extractable from the detector to the outside can be increased to improve the reading efficiency, and the high-speed responsiveness of the reading out and the high efficiency in extracting the signal charges can be combined with each other.
Incidentally, in the detector provided with the sub-stripe electrodes as described above, a width of each optical charge pair generating linear electrode, a width of each optical charge non-generating linear electrode, and a width between the linear electrodes greatly influence the reading efficiency of the stored charges.
For example, in the case where the width of the optical charge pair generating linear electrode and the width of the optical charge non-generating linear electrode are narrowed, there is a fear that the reading signals are delayed since electric resistances of these linear electrodes are increased and that the electrodes are disconnected due to an etching defect and the like during manufacturing thereof.
Moreover, in the case where the width between the linear electrodes is narrowed, there is a fear that an electric discharge occurs to short-circuit the respective linear electrodes when a high voltage is applied thereto and that the linear electrodes are short-circuited due to interfusion of dust and the like during manufacturing of the electrodes.
Furthermore, in the case where the width of the optical charge pair generating linear electrode, the width of the optical charge non-generating linear electrode and the width between the linear electrodes are widened, an optical path is elongated, in which optically generated charges erase the latent image charges stored in the capacitor unit. Therefore, the reading efficiency of the detector is reduced.
Meanwhile, a ratio of the width of the optical charge pair generating linear electrode and the width of the optical charge pair non-generating linear electrode also influences the reading efficiency of the stored charges greatly.
For example, suppose the case where the width of the optical charge pair non-generating linear electrode is widened more than the width of the optical charge pair generating linear electrode. Then, when the first conductive layer and the second conductive layer are short-circuited after irradiating the recording radiation thereto, the amount of charges induced to the optical charge pair non-generating linear electrode is increased. Therefore, signal detection efficiency is improved. On the other hand, since the width of the optical charge pair generating linear electrode is narrow, an amount of effective light during the optical reading is reduced, and a line resistance of the optical charge pair generating linear electrode is increased, and thus it becomes difficult to detect the signals.
Moreover, in the case where the width of the optical charge pair non-generating linear electrode is narrowed more than the width of the optical charge pair generating linear electrode, the width of the optical charge pair generating linear electrode is wide. Therefore, the amount of effective light during the optical reading is high, and the line resistance of the optical charge pair generating linear electrode is reduced, and thus it becomes easy to detect the signals. On the other hand, when the first conductive layer and the second conductive layer are short-circuited after irradiating the recording radiation thereto, the amount of charges induced to the optical charge pair non-generating linear electrode is reduced. Therefore, the signal detection efficiency is lowered.
As described above, unless the width of each linear electrode, the width between the linear electrodes and the ratio of the widths of the both linear electrodes are optimized, then various problems occur.